1. Field of the Invention
The present invention relates to a loop antenna used in a wireless communication apparatus.
2. Description of the Related Art
Wireless communication technology has recently received a great deal of attention, and even small apparatuses such as digital cameras are equipped with a circuit and an antenna for wireless communication. To equip a small apparatus such as a digital camera with a wireless communication circuit and antenna, the circuit and the antenna need to be smaller. For example, the antenna is implemented on a dielectric substrate to reduce cost and size.
Examples of related arts of a loop antenna with a parasitic element arranged near it include patent references 1 and 2. In patent reference 1, a parasitic element about ¼ the wavelength is arranged near the loop antenna, thereby broadening the communication frequency bandwidth. Patent reference 2 discloses three types of parasitic element shape. In the first shape, a parasitic element having an opening portion on the feeding side of the loop element is arranged to change the resonance frequency and improve the gain. In the second shape, a parasitic element having no opening portion is arranged to change the characteristic impedance. In the third shape, a window-shaped parasitic element is arranged to lower the resonance frequency.    [Patent Reference 1] Japanese Patent Laid-Open No. 2006-295545    [Patent Reference 2] Japanese Patent Laid-Open No. 09-148838
A high-frequency circuit in a wireless communication apparatus is generally designed to have a characteristic impedance of 50Ω. The input impedance of a loop antenna having a basic shape is 75Ω. For this reason, when the loop antenna is directly connected to the 50Ω a high-frequency circuit, impedance mismatch occurs, and no satisfactory characteristics can be obtained. Satisfactory characteristics can be obtained by a loop antenna whose input impedance is 75Ω. To convert the characteristic impedance of the high-frequency circuit of the wireless communication apparatus from 50Ω to 75Ω, an impedance conversion unit (balun) needs to be provided on the preceding stage of the input to the antenna.